In U.S. Pat. No. 3,638,211, there is described a crane safety system for warning the operator of a crane when the crane is about to overturn due to the moment of heavy load or when the weight of that load could cause structural failure of the crane. In that arrangement, sensors are provided for measuring the boom length, the boom angle, the condition of the crane support, and the quadrant in which the crane is operating. These sensors are connected to the crane and apply signals to the computer which selects previously stored information from a memory unit depending upon the signals received. This stored information is applied to a comparator which compares the stored signal against a measured load signal and provides a warning alarm to the crane operator when the two signals approach each other.
In this prior-art device, the memory unit is a single memory in which the highest permissible load moment is stored as a function of the crane parameters directly. The highest permissible load moment includes, as is known, the intrinsic moment of the crane boom, i.e. the portion of the load moment due to the weight and length of the boom, and the highest permissible load moment resulting from the application of various loads to the boom. This latter component can be treated as a loading moment and can be thought of as the moment resulting only from the presence of a load of given magnitude at the end of the boom. Since this load varies from time to time in the operation of the crane and, indeed, may be an unpredictable value, the total load moment which is applied to the crane boom is likewise indeterminate and depends upon the weight of the boom, its extension, the angle of the boom and the aforedescribed loading.
While the highest permissible load value, as a function of the crane parameters for various crane booms is constant for one and the same type of boom crane, the intrinsic load moment is thus the instantaneous or loaded moment for any given set of crane operating parameters has a specific value for each crane boom.
The crane parameters referred to above and hereinafter are generally the boom length and thus the boom extension, the angle of the boom in a vertical plane and the angle of the boom in a horizontal plane.
If it is desired to provide a setpoint signal of high precision for comparison for a measured-value for actual-value signal to provide the warning or to control the operation of the boom, e.g. by terminating the displacement thereof when the actual value signal approaches the setpoint signal, it has been the practice heretofore to provide for each individual boom-type crane a set of maximum permissible total load moment values as a function of these crane parameters by measuring the instantaneous total load moment, for example, of a lifting piston-cylinder arrangement of the crane boom for loads which are increased progressively in stages. These signals are stored in the single memory of the aforementioned U.S. patent or otherwise programmed therein.
Such measurements are time consuming and expensive. This is even the case when the highest permissible load values are already known because of variations in the intrinsic load moment of the crane booms from crane to crane. Usually these measurements must be taken over the wide range of capacity of the crane and over the wide ranges of operation of the boom with respect to the aforementioned angles and boom length. Naturally, if the maximum permissible load value varies from crane to crane because of different constructions of the crane boom, the number of measurements is inordinately increase.